Method of anodically polishing steel



Patented Apr. 18, 1944 METHOD OF ANODICALLY POLISHING STEEL Charles L. Faust, Columbus, Ohio, assigner to Battelle Memorial Institute, Columbus, Ohio, a

corporation of Ohio Application November 15, 1939, Serial No. 304,450

3 Claims. (Cl. 204-140) This invention relates to a method of and an electrolyte for anodically polishing plain carbon steel, that is, steels having a total of not over 6% of alloying ingredients, and low carbon, or substantially carbon free, iron. More particularly the invention relates to an aqueous electrolyte comprising phosphoric acid, chromic acid and Water and to the use of such electrolyte in the anodic polishing of plain carbon steels and irons.

I have now found that an aqueous electrolyte comprising phosphoric acid, chromic acid and water, as the essential ingredients, can be satisfactorily used in the anodic polishing of plain carbon steels and ingot iron. If the relative proportions of these essential ingredients of the bath be kept within certain limits, which I have deter-mined, very satisfactory polishes can be produced in the case of plain carbon steels and ingot iron, while at the same time producing surfaces having relatively superior characteristics to those obtainable by mechanical polishing or bufng operations.

it is therefore an important object of my invention to provide an electrolyte containing phosphoric acid, chromic acid and Water within certain limits as to their relative proportions, for

' use in the anodic polishing of plain carbon steels and ingot iron to impart thereto a highly lustrous surface.

It is a further important object of this invention to provide a method of anodically polishing plain carbon steels and ingot iron, using an electrolyte of novel composition that is operative throughout a wide range of anode current densities and temperatures.

Other and further important objects of this invention will become apparent from the disclosures in the specification and the accompanying drawing.

This invention (in its preferred form) is illustrated in the drawing and hereinafter more fully described.

On the drawing:

The figure represents a triaxal diagram showing the relative proportions of phosphoric acid, chromic acid and water in compositions of electrolytes embodying my invention.

In the 4accompanying diagram, the respective sides of the triangle indicate the percentages of phosphoric acid (HgPOi), of water (H2O), and of chromic acid (CrOa), from to 100%. On the basis of experimental data, I have determined the relative proportions of phosphoric acid, chromic acid and Water that give compositions of electrolyte that are operative for the anodic polishing of plain carbon steels and ingot iron. The area representing operative compositions of electrolyte is defined on the accompanying diagram by the solid lines AB, BC and CA. Within the area so defined, any composition selected will be found to be operative in the method hereinafter described for the anodic polishing of plain carbon steels and ingot iron.

In order to get the best polishing results, however, I have found that the range of proportions of phosphoric acid, chromic acid and Water should be kept within somewhat narrower limits and these narrower limits are represented on the accompanying diagram by the area defined by the solid line AE, dot and dash line ED, and solid line DA. The preferred compositions of electrolyte, with respect to the relative proportions of phosphoric acid, chromic acid and water, lie within this second area, which is wholly enclosed within the broader area first defined.

The reading of a triaxial diagram such as the accompanying ene is well understood, but the following will be given for purposes of illustration. The point on the diagram represented by the reference letter A, for instance, indicates a composition co-mprising about of phosphoric acid, a small but signicant proportion, say 0.1% of chromic acid and the balance, somewhat less than 15%, water; the point indicated by the reference letter B a composition comprising 63% of phosphoric acid, say 0.1% of chromic acid and the balance, somewhat less than 37%, water; the point indicated by the reference letter C a composition comprising 45% of phosphoric acid, 29% of chromic acid and 26% of water; and the point indicated by the reference letter D a composition comprising 53% olf phosphoric acid, 23% of chromic acid and 24% of water.

It will be seen that the maximum phosphoric acid content is about 85% and the minimum 45%; the maximum chromic acid content 29% and the minimum about 0.1%; and the maximum water content about 37% and the minimum about 14%. The preferred composition limits are from 53 to 85% phosphoric acid; from 0.1 to 23% chromic acid; and from 14 to 31% water. The relative proportions of phosphoric acid, chromic acid and water, however, are interdependent, so that points should be selected within the areas defined upon the accompanying diagram in order to insure satisfactory results.

While the accompanying diagram indicates relative percentages cf phosphoric acid, chromic acid and water in a system consisting of only these three components, bath compositions may nevertheless contain other ingredients, such as other acids and/or salts, and still be satisfactory for use in the polishing of plain carbon steels and ingot iron, provided that the relative proportions of the three essential ingredients,phosphorc acid, chromic acid and water lie withinrthe operative or preferred areas defined on the accompanying diagram.

Thus, for instance, in making up an electrolyte for the anodic polishing of plain carbon steels and ingot iron, one might select the point X on the triaxial diagram as being a bath of preferred composition. The composition represented by the point X would be 70% phosphoric acid, 10% chromic'acid and 20% water. During the continued use of such a bath in the electropolishing of plain carbon steels and ingot iron, the bath composition would necessarily change, owing to the anodic dissolution into the bath of the steel or iron undergoing polishing. There might also be some change in the Water content, such as an increase due to absorption by the bath of moisture from the air, or a decrease due to evaporation of water from the surface of the bath, to decomposition of Water by electrolytic action, or to loss of water from the bath by entrainment thereof in gases given off from the bath.

Notwithstanding such changes in its composition as may occur during continued use, if the relative percentages of phosphoric acid, chromic acid and water, expressed as percentages by weight of the total weight of only these three ingredients in the bath composition, remain within the area defined by the lines AE, ED and DA, the bath will continue to operate satisfactorily. Even if the bath composition be so altered during continued operation that it falls within the less preferred area defined by the lines EB, BC, CD and DE, on the accompanying diagram, the bath will continue to function, although not so satisfactorily.

Consequently, where the relative percentages of phosphoric acid, chromic acid and water in a given bath composition lie within either the preferred or less preferred areas dened on the accompanying triaxial diagram, such bath composition is intended to come within the scope of my invention, even though it may contain other acids than phosphoric and chromic and even though it may contain a substantial quantity of metallic salts.

Instead of chromic acid, soluble chromates and bichromates may be substituted therefor and are to be considered the equivalent of chromic acid on a stoichiometric basis. The term chromic acid equivalent, as used in this specification and in the claims, is therefore intended to include chromic acid itself (CrOs) and stoichometrically equivalent weights of soluble chromates and bichromates.

Similarly, in place of orthophosphoric acid, other phosphoric acids, such as metaand pyrophosphoric acids, may be used and are to be considered as included within the term phosphoric acid.

In the method of anodically polishing both plain carbon steels and ingot iron, using a b-ath of a composition indicated to be suitable by reference to the accompanying triaXial diagram, the plain carbon steel or ingot iron, or an article having a surface of such steel or iron, is made the anode in a bath of the selected composition and an electric current is passed therethrough of sufficient density and for a sufficient length of time to produce the desired high degree of luster, or polish, on the metal surface. By employing an electrolyte having a composition within the preferred area defined on the accompanying triaxial diagram, a highly lustrous, mirror-like surface can be readily obtained. The highly lustrous surface obtainable by my method, using an electrolyte of preferred composition, is an important feature of my invention and one that sharply distinguishes from the prior art finishes produced in the electrolytic cleaning, or bright pickling, of low carbon steels and ingot iron.

The formation of highly polished and lustrous surfaces is undoubtedly associated with the presence of a polarizing film over the surface of the metal during the process of anodic dissolution. The nature of this lm is such that selective attack on the various phases present in the plain carbon steels, ingot iron or the like, is minimized. Anodic dissolution apparently takes place at a relatively high rate and at a high anodic polarization value, with the result that the anodic dissolution of the metal acts to level the crystal surfaces thereof and to produce a mirror-like finish. These conditions do not prevail in the simple electrolytic cleaning treatment known to the prior art.

In order to obtain the best results in a reasonable length of time, it is preferable to use relatively high current densities, such as those of the order of magnitude of from 50 to 1000 amperes per sq: ft., but preferably between and 500 amperes per sq. ft. It will be understood that even lower current densities may be employed, down as low as about 10 amperes per sq. ft., but longer times of treatment will be required and the process may, for that reason, be uneconomlcal. Higher current densities, up to as high as 2000 or 3000 amperes per sq. ft., may also be used, but such high current densities imply, in general, larger current, which require more expensive equipment. The length of time to effect the desired results depends upon the magnitude of the current densities employed and to some extent upon the particular analysis of the low carbon steel, ingot iron or the like, to be polished, and also upon the character of the surface of the steel or iron initially. Rough surfaces, of course, require a longer time to polish than relatively smooth ones.

With any of the compositions of electrolyte lying within the preferred area defined by the lines AE, ED and DA, on the accompanying traxial diagram, excellent polishes are obtained on plain carbon steels and ingot iron, when the steel or iron is made the anode at current densities of 250 amperes per sq. ft. and the treatment carried out for a period of 30 minutes at a bath temperature of about F. In general, the temperature may be maintained at any point between room temperature and the boiling point of water, but temperatures of around 100 to 155 F. are found very satisfactory.

From the foregoing description Yof my invention, it will be apparent that I have provided a novel composition of electrolyte and a method whereby the same may be used to produce highly lustrous polishes on plain carbon steels and ingot iron. My electrolyte and method avoid the disadvantages of the old methods of mechanically polishing and enable the production of plain carbon steel and ingot iron articles having surfaces that are free from mechanical strain, dragging and piling and which are superior to those obtainable by mechanical polishing methods.

It will, of course, be understood that various details of the process may be varied through a wide range without departing from the principles of this invention and itis, therefore, not the purpose to limit the patent granted hereon otherwise than necessitated by the scope of the appended claims.

I claim as my invention:

1. The method of anodcally polishing plain carbon and low alloy steels and ingot iron which comprises making the steel or iron the anode in a solution comprising essentially from 45% to 85% of phosphoric acid, from 0.1 to 29% of CrOs, and from 14 to 37% of water, all of said percentages being by weight of said solution and the relative percentages of said three solution ingredients lying within the area defined on the accompanying diagram by the straight line AB, the line BC and the line CA, and passing an electric current therethrough of sufficient density and for a sufcient period of time to effect a polishing of said steel or iron.

2. The method of anodically polishing plain carbon and low alloy steels and ingot iron, which comprises making the steel or iron the anode in a solution comprising essentially from 53 to 85% of phosphoric acid, from 0.1 to 23% of CrOa, and from 14 to 31% of Water, all of said percentages being by weight of said solution and the relative percentages of said three solution ingredients lying within the area dened on the accompanying diagram by the straight line AE, the line ED, and the line DA, and passing an electric current therethrough of a sufficient density and for a sufficient period of time to effect a polishing of said steel or iron.

3. The method of anodically polishing plain carbon and low alloy steels and ingot iron, which comprises making the steel or iron the anode in a solution comprising essentially from 53 to 85% of phosphoric acid, from 0.1 to 23% of CrO3, and from 14 to 31% of water, all of said percentages being by weight of said solution and the relative percentages of said three solution ingredients lying within the area defined on the accompanying diagram by the straight line AE, the line ED and the 1ine DA and passing an electric current therethrough of between and 500 amperes per square foot at a bath temperature between 100 and F. for a suflicient period of time to effect a polishing of said steel or iron.

CHARLES L. FAUST. 

